Recently, so-called tandem type image forming apparatus having a plurality of image forming portions have been put into practical use in order to increase printing speed.
FIGS. 5 to 7 show an example of such a tandem type full color image forming apparatus, which constitutes a background of the present invention.
FIG. 5 is a cross sectional view of the apparatus. The apparatus shown in FIG. 5 is provided with photosensitive drums 20 for respective colors of yellow, magenta, cyan and black. Each photosensitive drum 20 is composed of an electrically conductive member coated with a photosensitive layer, on which an electrostatic latent image is to be formed by a laser beam emitted from a scanning type optical device. Reference numeral 21 designates the scanning type optical device that emits a laser beam based on image information sent from an image reading apparatus or a personal computer etc. (none of which is shown in the drawings), reference numeral 22 designates a developing device for forming a toner image on the photosensitive drum with triboelectrically charged toner, reference numeral 23 designates an intermediate transfer belt for transferring the toner image on the photosensitive drum onto a transferring sheet, reference numeral 24 designates a feed cassette that accommodates paper sheets on which toner images are to be formed, reference numeral 25 designates a fixing device for causing the toner image having been transferred on the sheet to adhere to it with heat, and reference numeral 26 designates a delivery tray on which the transferring sheet on which the image has been fixed is to be stacked.
In the following, the image forming process will be described.
A laser beam emitted from the scanning type optical device 21 in accordance with certain image information is radiated onto the photosensitive drum 20, so that an electrostatic latent image is formed on the photosensitive drum 20, which has been charged by a charger 27. Then in the developing device 22, triboelectically charged toner is applied to the electrostatic latent image, so that a toner image is formed on the photosensitive drum. The toner image is transferred from the surface of the photosensitive drum onto the intermediate transfer belt 23. The toner image is then further transferred from the intermediate transfer belt 23 onto a paper sheet having been transported from the feed cassette 24 provided in the lower portion of the apparatus main body. Thus, an image is formed on the paper sheet. The image formed on the paper sheet is fixed (i.e. subject to toner fixing) by the fixing device 25, and the sheet is delivered to be stacked onto the delivery tray.
FIG. 6 is a diagrammatic sketch showing an image forming portion that constitutes a part of the image forming apparatus shown in FIG. 5. The scanning type optical device shown in FIG. 6 forms an electrostatic latent image on the photosensitive drum by means of a polygon mirror 29 for deflecting the laser beam, which is emitted based on image information, in a scanning manner, f? lenses 30a and 30b for enabling constant speed scanning with the laser beam and for imaging the laser beam as a spot on the photosensitive drum, and a turn-back mirror 31 for reflecting the beam toward a predetermined direction. The scanning type optical device is vulnerable to dust and soil, and if some part in the optical path of the laser beam is soiled or contaminated, problems such as an image defect at the portion of an image that corresponds to the soil or insufficiency in toner at the portion of the image. In view of such problems, Moltopren (trademark) has been used to provide a sealing between an optics case 33 that accommodates optical parts and a top lid 34 as a countermeasure against entrance of dust. On the other hand, with the downsizing of the image forming apparatus main body, the location of the scanning type optical device has been made closer to the photosensitive drum. In addition, in the apparatus of this example, constituent parts are so arranged that the scanning type optical device is disposed below the drums in order to facilitate maintenance of portions in the circumference of the photosensitive members.
However, in the case in which the laser beams are radiated from beneath the photosensitive drums or in the case in which the laser beams are radiated from positions close to the photosensitive drums, toner scatters or drops from the developing device, the photosensitive drums or a cleaner etc. in the directions indicated by arrows in FIG. 6, as the apparatus is used.
In view of this, it is necessary to provide dust-proof glass plates 32 at laser beam emitting portions, as shown in FIG. 7, in order to prevent entrance of dust into the interior of the optics case and to prevent contamination of the optical parts.
However, in the image forming apparatus in which the optics case is disposed beneath the apparatus as described above, no countermeasure against contamination of the dust-proof glass plate 32 with dust or toner has been taken, though contamination of the optical elements accommodated in the optics case with dropping toner can be prevented from occurring. Therefore, as shown in FIG. 7, falling toner would slip down on the top surface of the optics case to eventually contaminate the surface of the dust-proof glass plate 32 over time. Such contamination would block the laser beam partially to prevent formation of the electrostatic latent image on the photosensitive drum, which would result in generation of a defective image.